FIG. 1 shows a conventional chopper power supply of the half-bridge type. A chopper power supply of that type is described, in particular, in "Les alimentations a fr equence de d ecoupage elev ee" ["Power supplies having high chopper frequencies"] by Daniel Sadarnac, ESE 9, Eyrolles.
A voltage source supplies a DC voltage E to a bridge arm including two switching sections 10 & 11 in series. Switching section 10 comprises the following connected together in parallel: switching means T1, a protective diode D1, and a capacitor C1. Likewise, switching section 11 comprises the following, also connected together in parallel: switching means T2, a protective diode D2, and a capacitor C2. The midpoint P of the bridge arm shown is connected to an inductor L1 followed by a load, constituted in this example by a transformer having a primary inductor L2 and a secondary inductor L3. A capacitor C3 is connected in parallel to the primary inductor L2. The other end of the load constituted by inductor L2 is connected between two maintaining capacitors C4 and C5, to which the DC voltage E is applied.
At the secondary winding of the transformer, two rectifying diodes D3 and D4 co-operate with two inductors Ls and a smoothing capacitor Cs to supply a DC output voltage Vs. Voltage Vs is applied to a load (not shown).
Operation of that power supply is described with reference to FIG. 2 which shows four corresponding timing diagrams (A to D) of the signals shown in FIG. 1.
For example, the switching means T1 and T2 are constituted by MOSFET power transistors including the protective diodes D1 and D2. A control circuit (not shown) applies control pulses shown in timing diagrams A and B to the gates of the transistors T1 and T2. The pulses have a period T and are offset in time so as to switch the transistors T1 and T2 on (so that they are saturated) and off (so that do not conduct) in alternation. When a pulse is applied to the gate of one of the transistors, it is switched on. Such a pulse is applied at time t1 to transistor T1, with transistor T2 not conducting. The voltage e (timing diagram C) measured between the midpoint between capacitors C4 and C5 and the midpoint of the bridge arm is equal to +E/2, which is the voltage present across the terminals of capacitor C4. The current i (timing diagram D) flowing through transistor T1 increases and is supplied to the load constituted by inductor L1 and by the inductor L2 of the primary winding of the transformer. At time t2, transistor T1 is switched off and inductor L1 prevents the current from passing through rapidly, thereby charging capacitor C1 and discharging capacitor C2. The voltage e then decreases to -E/2 at time t3, when diode D2 starts conducting. Capacitor C2 then presents a low voltage across its terminals. A guard time dt is provided between the end of conduction of one transistor and the start of conduction of the other transistor so as to prevent the capacitors C1 and C2 from being suddenly charged and discharged. In this way, losses are decreased. Once the voltage e has become negative, the current i is reversed and, at time t4, transistor T2 is switched on. The phenomenon is then repeated symmetrically.
In this way, a symmetrical AC voltage e is generated which makes it possible to obtain an output voltage Vs that is a function of the chopper period T.
The capacitance of the capacitors C1 and C2 results from a compromise between the losses caused, and the chopper frequency. If their capacitance is large, switching losses decrease but the chopper frequency must be relatively low because it takes more time to charge them and to discharge them. Furthermore, their presence is essential in order to perform low-voltage switching, also referred to as "soft switching".
The main drawback of a chopper power supply of that type is that it is essential for inductor L1 to be used in order to enable the capacitors C to be charged and discharged. When the load is constituted by a transformer, its primary inductance is too low to make it possible to generate a high enough charging current and a high enough discharging current for the capacitors C1 and C2. At the capacitor charge and discharge instants, the current i must be high to enable fast charging and fast discharging. In this way, with reference to FIG. 2, the current must be high enough during the periods t2 to t3 and t5 to t6, but it is also high the rest of the time while it is flowing through the transistors T1 or T2 or through the diodes D1 or D2, and this gives rise to conduction losses and to switching losses. The transistors must therefore be over-dimensioned.
Moreover, if the impedance presented by the load powered by that apparatus decreases, the current i must nevertheless be present and must be high enough at the switching instants. In the absence of load, losses are therefore high.
Another problem posed by inductor L1 is that it is difficult to construct, and it gives rise to high iron losses. It is necessary to implement that inductor in the form of a torus for reasons of electromagnetic radiation, and, to prevent it from overheating, it must be over-dimensioned, which makes the apparatus less compact. In high-current applications, e.g, for a 500 W chopper power supply, the inductance of L1 must be approximately equal to a few .mu.H and it must be capable of passing 15 A. It can be considered that the presence of that inductance gives rise to a loss of efficiency of about 3%. Since iron losses increase with f.sup.3/2, that apparatus is also limited to chopper frequencies of less than 1 MHz.
Furthermore, when powering a load that varies, or if the voltage E is not fixed, the output voltage Vs can be kept constant only by servo-controlling the chopper frequency. This increases the overall size of the apparatus, and the cost thereof, and poses problems related to electromagnetic radiation.